This invention relates to an apparatus for measuring and controlling the flow rate of a fluid in a conduit of the type having first and second control valves in the conduit with a flow sensing element of the differential pressure type located in the conduit between the control valves. A flow rate indicating means is connected to the flow sensing element through high pressure and low pressure conduits and includes apparatus for detecting and indicating fluid leakage past a valve into the isolated portion of the fluid conduit between the control valves.
Fluid flow measuring and controlling apparatus are commonly used in a diversity of applications ranging from the control and measurement of the flow rate of cryogenic liquids to the control and measurement of the flow rate of high temperature gases, and in some cases, supercritical fluids. In certain of these applications, it is highly desirable for safety or process reasons to be able to insure positive shut-off of the fluid at the control valves and to have a positive indication of any leakage past the valves. Such conditions occur in applications involving the use of toxic or highly reactive fluids, or those where leakage of the fluid past the shut-off valve would create hazardous conditions or would damage equipment or materials downstream of the flow measuring and controlling equipment. Examples of such applications include the supply of oxygen or other gases to various processes; the supply of steam to reactions; or the supply of various reactants to a reaction vessel. A specific example of the application of this invention is to the control and measurement of the supply of oxygen gas for enrichment of air used in cupola furnaces.
In the prior art, fluid leakage past the first control valve when both the first and second valve are in shut-off position has been detected by means of a vent conduit communicating directly with the fluid conduit at a point between the first and second control valves through which the leakage fluid discharged. Such vent conduits included valves opened manually or in response to a signal from a remote location. In other applications the vent conduit included a pressure relief valve which opened to discharge the leakage fluid when the pressure in the fluid conduit between the first and second control valves exceeded a predetermined pressure, generally higher than the normal working pressure of the fluid downstream of the first shut-off valve. Detection of the leakage was accomplished by inclusion of a flow indicator in the vent conduit on the discharge side of the valve. Alternatively, a remote alarm would be actuated by the opening of a relief valve. A further method disclosed by the prior art of detecting leakage past the valve seats of a shut-off valve is disclosed in U.S. Pat. No. 2,430,122, wherein an extraneous fluid is admitted to the body of the valve between the upstream and downstream valve seats and any leakage past the valve seats is detected by flow of the extraneous fluids through the supply conduit.
In order to provide a positive indication of leakage past the first valve in prior art flow measuring and controlling apparatus, connection of a vent conduit to the fluid conduit in addition to the flow measuring apparatus between the first and second valves is required. A second flow indicator, such as a rotameter, was also required on the vent conduit in addition to the flow rate indicator connected to the flow sensing element in the fluid conduit. Often the flow rate indicator is located remotely from the fluid conduit at a control panel. In the prior art, the separate vent conduit involved significant additional cost in installing such vent conduit to the control panel where the leakage indicator was located, as well as the cost attendant with a second flow indicator.